1. Field of the Invention
The present invention relates to the detection of defects in patterned substrates, such as semiconductor wafers, by inspection using a charged particle beam. More particularly, the present invention relates to improving the uniformity and voltage contrast of an image produced by a charged particle beam inspection tool.
2. Description of the Related Art
Defect detection is an important aspect in the manufacture of semiconductor devices. Early detection, preferably at multiple stages of fabrication, enables a source of defects to be identified and eliminated before large numbers of wafers are affected. Currently, the majority of in-line inspection is performed using optical inspection tools, such as the 21 XX-series wafer inspection tools from KLA-Tencor. These optical tools, however, are limited in their capabilities by their small depth of focus and blurring due to diffraction. The small depth of focus of these optical tools is an inherent limitation of the large numerical aperture objective lenses required to image sub-micron features. Any defect that is not at the surface of the device will be substantially out of focus and therefore undetectable. Examples of such sub-surface defects include polysilicon gate shorts, open vias and contacts, and metal stringers. In addition, the diffraction-limited resolution of optical tools blurs small surface defects rendering them undetectable as minimum critical dimensions (CDs) shrink below 0.25 xcexcm. These include defects such as xcx9c0.1 xcexcm particles and regions of missing or extra pattern which are at or below the minimum CD.
Charged particle beam inspection will likely become one of the critical technologies in advanced semiconductor manufacture. Charge particle beam inspection tools, which include conventional scanning electron microscopes (SEMs), focused ion beam microscopes (FIBs) and electron-beam (E-beam) defect detection systems, have a much higher resolution than optical tools and are able to detect smaller size defects. E-beam defect detection systems can also detect sub-surface defects by measuring the voltage contrast change resulting from the electrical effect of killer defects, i.e., xe2x80x9copenxe2x80x9d and xe2x80x9cshortxe2x80x9d type defects. See, for example: T. ATON et al., Testing integrated circuit microstructures using charging-induced voltage contrast, J. VAC. Sci. TECHNOL. B 8 (6), November/December 1990, pp. 2041-2044; K. JENKINS et al., Analysis of silicide process defects by non-contact electron-beam charging, 30TH ANNUAL PROCEEDING RELIABILITY PHYSICS 1992, IEEE, March/April 1992, pp. 304-308; J. THONG, ED., ELECTRON BEAM TESTING TECHNOLOGY, Pelnum Press 1993, p.41; and T. CASS, Use of the Voltage Contrast Effect for the Automatic Detection of Electrical Defects on In-Process Wafers, KLA Yield Management Seminar 1996, pp. 506-2 through 506-11.
Schlumberger""s E-beam defect detection technology operates in either a positive or negative voltage contrast mode. In either mode, floating electrical conductors on the wafer under inspection are raised to a potential by pre-charging the surface of the wafer with charged particles (e.g. electrons). Because they appear in different contrasts, the floating and grounded connectors can therefore be distinguished. In a positive voltage contrast mode the floating conductors are charged to a more positive voltage than the grounded conductors, while in a negative voltage contrast mode the floating conductors are charged to a more negative voltage. A focused, low voltage particle (electron) beam interrogates the charge states of the wafer""s conductors. By comparing the voltage contrast image (or partial image) of a die with that of a reference (e.g. a neighboring die), one can locate defects in the die. Because this technique relies on voltage contrast variation to identify defects, it is important to have: (1) a uniform voltage contrast image in which the background contrast is uniform; (2) a consistent contrast for a circuit when that circuit is located in different areas of the field of view; and (3) a distinctive contrast (e.g., a large difference) between circuit elements with different underlying connections.
One problem with charged particle beam inspection systems is that the resulting images are often non-uniform in quality. Unwanted variations in the topographic contrast or voltage contrast of an image often exist. Non-uniformity in the voltage contrast can result from uneven charging of a patterned substrate (wafer or die). Surface charging can affect secondary electron collection efficiency and the on-going charging process during primary beam irradiation. E-beam defect detection systems operate between two crossover voltages, at which a primary electron induces more secondary electron emission current than primary current. This means that floating conductors within a field of view (FOV) will charge positively. Uncaptured secondary electrons which are returned to the wafer can negatively charge the area surrounding the FOV, thereby creating a xe2x80x9cmicroxe2x80x9d retarding field (MRF). The MRF affects the surface charging process and can cause several problems with the voltage contrast of an image. First, the MRF can cause some secondary electrons to be rejected back into the FOV area of the wafer, thereby reducing the positive voltage contrast. Second, if the magnification of the system is increased for detailed inspection, the MRF can cause a positive voltage contrast mode to switch to negative. At a high magnification, a strong MRF will retard back enough secondary electrons back to the FOV area, thereby negatively charging the FOV area. Third, the MRF can create unpredictable xe2x80x9cghost featuresxe2x80x9d and site dependent contrast variations in an image. The MRF is non-isotropic at the edge of the FOV, and the intensity of returned secondary electrons at the edge of the FOV can differ significantly from those at the center of the FOV. This results in uneven charging of floating structures. In addition, the efficiency of detecting secondary electrons emitted from the center and the edge of the FOV can differ greatly. These problems produce false contrast differences which greatly degrade the reliability of E-beam defect detection systems.
Since 1995, Schlumberger has used E-beam probers such as the commercially available IDS 10000 system on passivated integrated circuits (ICs) to measure waveforms at a high beam current. The E-beam prober scans a large area and then images a smaller area. A high current vectored beam is pulsed to measure capacitive AC waveforms on the passivated IC. Imaging the small area prior to scanning the large area reduces the unstable surface charging in the small area, thereby producing a more stable and accurate voltage waveform (as a function of time). The uniformity or contrast of an image is not a concern because the measurement is taken at the area of a conductor on an individual die. This method is only applicable to functioning integrated circuits connected to an electrical stimulus, rather than to unfinished patterned substrates.
It is also known to try to improve measurements produced by charged particle beam tools. International Application No. PCT/US98/00782 published on Jul. 23, 1998 as International Publication No. WO 98.32153 is directed to measuring critical dimensions of microcircuits using SEMs. Multiple scans of a SEM over a small scan area result in dark images, obscuring the features of the area. Scanning a larger area brightens the image. This method, however, merely brightens an image rather than enhances the image contrast differences between features with different underlying connections. In addition, simply brightening an area will not improve the uniformity of an image.
Accordingly, there is a need to improve the uniformity and contrast quality of an image produced by a charged particle beam inspection tool, in order to enhance the detection of defects on a patterned substrate. In particular, it would be desirable to enhance the voltage contrast of the image.
In accordance with one embodiment of the present invention, a method for detecting defects in a patterned substrate includes directing a charged particle beam onto the substrate, scanning the beam across the substrate, and optimizing parameters of the beam to improve the uniformity and contrast of a resulting image. The defect detection is undertaken when the device is not completely fabricated. The method further includes acquiring at least a partial image of a first area of the substrate. This step includes charging a second area of the substrate and imaging the first area. The second area encompasses the first area. The acquired image is then compared with a reference (e.g., by human inspection of the image or automatically with a processor) in order to identify any defects in the patterned substrate.
The voltage contrast quality of the image may be optimized by adjusting any one or more of a scan area size, scan speed, beam dose, beam current, beam energy, beam spot size (e.g., by de-focusing the lens), wafer chuck bias voltage, charge control plate bias voltage, energy filter voltage, and scan direction relative to a circuit pattern. The settings for scanning the first and second areas need not be the same.
In accordance with a further embodiment, a method of optimizing voltage contrast quality uses the charge control apparatus as stated in U.S. patent applications Ser. No. 08/892,734 filed Jul. 15, 1997, and Ser. No. 08/782,740 filed Jan. 13, 1997, and U.S. Pat. No. 6,091,249 issued Jul. 18, 2000. An electric field perpendicular to the wafer surface can be generated by biasing two electrodes that sandwich the wafer in order to control surface charging during the image scan (small area scan) and pre-charge scan. The field strength used during the imaging and pre-charging scans can be different. The voltage contrast image can also be enhanced by adjusting the energy filter voltage during the small scan following conventional E-beam prober practices.
In accordance with another embodiment, a method for detecting defects in a patterned substrate includes directing a charged particle beam onto the substrate, scanning the beam across the substrate, and optimizing parameters of the beam to improve a resulting voltage contrast image. Optimizing the beam""s parameters includes generating a performance matrix. The method further includes charging a first area of the substrate with a flood beam from a flood gun and interrogating a second area, encompassed by the first area, of the substrate, with a focused beam from a primary gun to acquire a voltage contrast image of the second area. The acquired voltage contrast image is then compared with a reference to identify any defects in the patterned substrate.
In accordance with a further embodiment, a method of optimizing a voltage contrast image and a speed of image acquisition includes determining upper and lower limits for parameters controlling a charged particle beam. The upper limits define an upper first area size and dose, at which the best voltage contrast image is produced, and the lower limits define a lower first area size and dose, at which the speed of image acquisition is optimized. The best voltage contrast image has a large contrast difference between circuits with different underlying connections. In addition, the voltage contrast is uniform with consistent contrast across the field of view for circuits of the same underlying connections. The method further includes creating a performance matrix indicating a voltage contrast quality and a time required to perform a charge operation at each first area size and dose, and then selecting from the performance matrix a particular first area size and dose based on the desired voltage contrast quality.
In accordance with still another embodiment of the present invention, an apparatus for detecting defects in a patterned substrate includes a charged particle beam column, a detector and at least one processor. A charged particle beam irradiates the patterned substrate to charge a first area of the substrate and to scan a second area, smaller than the first area, to acquire an image of the second area. The scanner operates at a high amplitude when the charged particle beam charges the first area and at a lower amplitude when the charged particle beam scans the second area. The charged particle beam charges the first area prior to scanning the second area to produce an image with uniform contrast. The detector detects secondary electron signals from the substrate. These signals are used to form the image of the second area, and the at least one processor compares the acquired image with a reference to identify defects in the patterned substrate.
In accordance with another embodiment, an apparatus for detecting defects in a patterned substrate includes a charged particle beam column, a mechanical stage, a detector, and at least one processor. The stage positions the patterned substrate relative to the column. The column includes a flood gun such as an in-the-lens flood gun, a primary gun, and a scanner such as a large field of view lens with main field and sub-field capabilities. The flood gun emits a flood beam for charging a first area of the substrate, and the primary gun emits a focused beam for scanning a second area., less than the first area, to acquire an image of the second area. The flood beam charges the first area before the focused beam scans the second area to produce an image with uniform voltage contrast. The beam scanner operates at a high amplitude when the flood beam charges the first area and at a lower amplitude when the focused beam scans the second area. Secondary electron signals detected by the detector are sent to a frame grabber to form an image of the second area. In case of a large field of view objective lens, multiple images can be acquired on different parts of the substrate before the stage need be moved. By pre-charging an area before moving to another area, one can avoid artifacts caused by imaging the previous area. More than one image can be acquired between each pre-charge step by repeating the above-mentioned steps. The at least one processor compares the acquired image with one or more references to identify defects in the patterned substrate.
The present invention, therefore, provides both a method and apparatus for detecting defects in an incomplete patterned substrate by inspecting the conductive or semi-conductive regions, such as contacts or unfilled vias, of the substrate. In particular, the invention improves the uniformity and contrast of an image which facilitates the detection of defects. A good voltage contrast image has a uniform background contrast throughout the image, a consistent contrast for a circuit when located in different areas of the field of view, and a distinctive voltage contrast between devices of different underlying connections. The comparison of a good voltage contrast image with a reference (e.g., another die or a previously stored image) is less likely to result in false defects.